Magnitude-4.3 shock strikes on 30 Jan 2016 near Helena, Montana

On 30 January 2016, a M=4.3 shock struck at a shallow 5 km (3 mi) depth in central Montana. The shock was felt in nearby Lincoln, and in Helena, 40 miles to the southeast. There are only sporadic mapped faults in the region, the closest being the Hilger fault to the southwest, visible in the Temblor map below. This ‘normal’ fault accommodates northeast-southwest continental stretching, but since earthquake scarps over the past 10,000 or so years are not evident in the landscape, the Hilger fault probably has a low slip rate (<0.2 mm/yr). As a result, the seismic hazard rank is quite low, perhaps deceptively so.

Temblor map showing the M=4.3 event and its aftershocks, with the Hilger and Helena Valley faults to the east. There have also been some M≤3 shocks most likely associated with the Hilger fault during the past month.

That is because the broad Intermountain Seismic Belt, within which the quake struck, is quite active, as can be seen in the map below. The Belt stretches from Kalispell to the north to Helena, Bozeman, and into Yellowstone National Park. The largest shocks in this belt are the 1959 M=7.3 Hebgen Lake, MT, shock just outside Yellowstone in the bottom center of the map, and the 1983 M=6.9 Borah Peak, ID, shock near the bottom left corner of the map. Unlike the M=4.3 shock, both of these earthquakes ruptured long faults that had recent scarps, and so it is unclear whether a M~7 shock is possible in the vicinity of the M=4.3 event. Nevertheless, there may be undiscovered faults that link those features that have been mapped.

Map of the seismicity of Montana from 1982 to 2000, with M>2.5 shocks in yellow, and M>5.5 shocks as orange stars. Source: Montana Bureau of Mines and Geology